Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to individually control the opening timing and closing timing of the intake valve and shifting the closing timing of the intake valve in a direction away from intake bottom dead center and raising the mechanical compression ratio the lower the engine load (for example, see Japanese Patent Publication (A) No. 2002-285876). In this internal combustion engine, at the time of idling operation, the intake valve is made to open after intake top dead center has been considerably past and is made to close after a short opening time.
Further, generally speaking, in an internal combustion engine, the lower the engine load, the worse the thermal efficiency, therefore to improve the thermal efficiency at the time of vehicle operation, that is, to improve the fuel consumption, it becomes necessary to improve the thermal efficiency at the time of engine low load operation. However, in an internal combustion engine, the larger the expansion ratio, the longer the period during which a force acts pressing down the piston at the time of the expansion stroke, therefore the larger the expansion ratio, the more the thermal efficiency is improved. On the other hand, if raising the engine compression ratio, the expansion ratio becomes higher. Therefore to improve the thermal efficiency at the time of vehicle operation, it is preferable to raise the mechanical compression ratio at the time of engine low load operation as much as possible to enable the maximum expansion ratio to be obtained at the time of engine low load operation.
However, if raising the engine compression ratio, the combustion chamber volume at intake top dead center becomes smaller. Accordingly, there is the problem that if opening the intake valve too much in advance of intake top dead center, the intake valve will end up interfering with the top of the piston. Therefore, when raising the mechanical compression ratio, it is necessary to prevent this problem from arising by making the intake valve open in a noninterference region where the piston is not interfered with. In this case, when opening the intake valve after intake top dead center, ordinarily the intake valve will not interfere with the piston. The intake valve interferes with the piston when the intake valve is opened before intake top dead center. Therefore, to prevent the intake valve from interfering with the piston, the intake valve must be made to open in the noninterference region before intake top dead center or must be made to open after intake top dead center.
However, in this case, if making the intake valve open after intake top dead center, the inside of the combustion chamber becomes a vacuum pressure until the intake valve opens and accordingly pumping loss occurs. Therefore, like in the above known internal combustion engine, at the time of idling operation, considerable pumping loss occurs when making the intake valve open considerably after intake top dead center.
Now, as explained above, to improve the thermal efficiency at the time of vehicle operation, it is preferable to obtain the maximum expansion ratio at the time of engine low load operation by making the mechanical compression ratio as high as possible. However, at this time, pumping loss occurs and the thermal efficiency ends up falling, so the meaning of raising the mechanical compression ratio ends up being halved.